Resonance Raman studies of the mechanism of photoreduction of iron protoporphyrin‐IX dimethyl ester in the presence of axial ligands

Abstract

AbstractResonance Raman studies of iron protoporphyrin‐IX dimethyl ester chloride [FeIII(PPDME)Cl] in the presence of biologically relevant axial ligands such as 2‐methylimidazole (2‐MeIm) and 1,2‐dimethylimidazole (1,2‐Me2Im) revealed the occurrence of photoreduction even under aerobic conditions by excitation near the Soret absorption band. A comparison of the vibrational modes sensitive to the oxidation, spin and coordination states of the photoreduced and chemically reduced forms of FeIII(PPDME)Cl confirmed that the reduction of the central iron atom in the high‐spin state takes place without any reduction of the porphyrin ring. No photoreduction is observed in pure dichloromethane solvent but trace amounts of alcohols and also some other polar solvents, which facilitate the separation of ion pairs, catalyse the process of photoreduction. From the exponential dependence of the fraction of sample reduced on the rate of photoreduction and illumination time, the rate constant for photoreduction, k−1R = 10 min, is obtained and the action spectrum of photoreduction shows a maximum coinciding with the Soret absorption band. The electron transfer from the axially ligated 2‐MeIm to iron appears to be a primary step in photoreduction. From the dependence of photoreduction on the concentration of 2‐MeIm, the ligand‐free, intermediate spin FeII (PPDME) was identified as the intermediate species formed during photoreduction.